1. Field of the Invention
This application relates to running control devices for watercraft, and, more particularly, relates to running control devices to control an angle of a propulsion device relative to a hull of a watercraft.
2. Description of the Related Art
A small watercraft is propelled by a propulsion device, such as, for example, an outboard motor mounted on the stem of the hull of the watercraft. The outboard motor (or other propulsion device) includes a thrust generating element (e.g., a propeller) mounted on a lower portion of the outboard motor. The outboard motor is advantageously provided with a tilt angle adjusting device to enable adjustment of an angle of the outboard motor relative to a hull. The tilt angle adjusting device comprises, for example, a tilt cylinder and a trim cylinder. The outboard motor is rotated to a large extent by the tilt cylinder between a service position (e.g., an operating position) and a non-service position (e.g., a non-operating position). When the outboard motor is in the non-service position, the outboard motor is lifted so that the thrust-generating element is exposed above the water surface. When the outboard motor is in the service position, the thrust-generating element on the lower portion of the outboard motor is submerged in a body of water. When the outboard motor is in the service position, the trim angle of the outboard motor relative to the hull is adjusted by the trim cylinder. The trim angle is advantageously adjusted in response to variations in operating conditions when the outboard motor is propelling the watercraft.
Generally, when accelerating the small watercraft up to a maximum velocity, the device for adjusting the trim angle is adjusted in accordance with the running condition of the watercraft. For example, the trim is controlled to be full-trim-in (e.g., in a state of minimum trim angle) until the watercraft begins planing, and the trim is controlled to be full-trim-out (e.g., in a state of maximum trim angle) after the watercraft starts to plane. Since air resistance increases in the higher velocity range, the velocity of the watercraft can be increased after planing has started by decreasing the trim angle a small amount to lower the bow so that the forward projection area is decreased. Similarly, a more efficient fuel consumption may be effected by controlling the trim angle for a maximum velocity at a throttle opening less than wide-open throttle (WOT). In such cases, the trim angle should be adjusted to be appropriate in response to the output of the outboard motor, the shape of the hull, the position of the center of the gravity (e.g., based on the number and locations of passengers and the weight and locations of cargo), and the like. Typically, the watercraft operator adjusts the trim angle appropriately by hand whenever necessary; however, manual trim angle adjustment is very troublesome for the operator, even if the driver is a veteran (e.g., experienced) operator. Thus, automatic adjusting methods of the trim angle have been suggested. See, for example, JP-A-S61-12498 and U.S. Pat. No. 5,167,456.
Conventional trim angle automatic adjusting methods in which the trim angle is adjusted while throttle opening is kept constant, have problems. For example, velocity adjustment is difficult. Even if a driver adjusts the throttle opening to obtain a target velocity, adjustment of the trim angle causes increased velocity so that the throttle opening should be readjusted.
As another example, difficulties in turning may be caused when a trim angle is adjusted for a maximum velocity at a throttle opening. In particular, the trim angle is generally adjusted outwardly. If turning is performed in this condition, a noticeable velocity drop may occur. This increases the difficulty of controlling the throttle to maintain the planning state of the watercraft. In some cases, cavitation (e.g., a phenomenon of the propeller racing in the body of water) may occur, which could damage the outboard motor. Furthermore, after a turn is completed the throttle opening should be readjusted to maintain the desired watercraft velocity.